The new design is meant to address concerns that the original plan would encroach on, and potentially damage, the La Brea Tar Pits at the neighboring Page Museum, casting a shadow over the largest pit.

Zumthor has said that the early models were always subject to revision. In any case, his updated design for the $650-million project, images of which LACMA released on Tuesday, makes clear that he took the criticism seriously.

He has significantly shrunk the footprint of the museum on the north side of Wilshire, leaving plenty of breathing room around the tar pits.

To make up for that lost space, the museum is now proposing to extend the new wing across the boulevard, where it will touch down on property owned by the museum at the southeast corner of Wilshire and Spaulding Avenue, which is now used as a parking lot.

Mark Boster / Los Angeles Times

Officials at the Page Museum greeted the new LACMA expansion design with relief that it no longer impinges on the La Brea Tar Pits. The original plan would have cast a shadow over the largest pit.

Officials at the Page Museum greeted the new LACMA expansion design with relief that it no longer impinges on the La Brea Tar Pits. The original plan would have cast a shadow over the largest pit. (Mark Boster / Los Angeles Times)

About one-quarter of the Zumthor building's 410,000 square feet would be contained on the Spaulding site, LACMA Director Michael Govan said Tuesday.

The decision to span a major thoroughfare like Wilshire is a bold one, to be sure. Drivers would pass under the building as they drove east or west along the boulevard.

At the same time, the new location will change the character of the building in ways that Zumthor has only begun to grapple with.

Govan has already won the support of city and county officials for the modified plan. Los Angeles Mayor Eric Garcetti has backed the idea of spanning Wilshire.

Los Angeles City Councilman Tom LaBonge, who represents the Miracle Mile area where the museum is located, said the new design offers a "tremendous vision" for the Museum Row section of Wilshire Boulevard. "It retains the historic beauty of the La Brea Tar Pits and at the same time crosses the boulevard in a way that will make it the center of the universe of art."

LaBonge said the Wilshire corridor will see other dramatic changes in coming years, with visitors coming to the museum via a subway station at Fairfax Avenue. "There's going to be an opportunity for people across the region to connect [to LACMA] through transit."

County Supervisor Zev Yaroslavsky, whose district includes LACMA, also praised the updated design.

"It solves the tar pit problem, and it creates a unique structure in Los Angeles," he said. "It will be a magnet not only to people in Los Angeles, but to people from around the world."

Jane Pisano, director of the Natural History Museum, which runs the Page, said in a phone interview Tuesday that she was also pleased with the changes.

Yet in trying to produce a more neighborly building, Zumthor and Govan have created some architectural challenges for themselves. And it's unclear at this stage quite how they plan to surmount them.

When it was contained on the central LACMA campus — north of Wilshire and east of museum buildings by Renzo Piano — the Zumthor design, especially as seen from above, had an unusual, even singular, architectural power.

Suggesting the work of artist Jean Arp and architect Oscar Niemeyer along with the oozing shape of the tar pits, Zumthor's black blob was a muscular graphic form. It would have floated on its expansive site — newly cleared with the proposed demolition of four existing LACMA gallery buildings — like an all-black abstract painting on a wide canvas.

Shifting the building above and across Wilshire fundamentally changes this equation, this relationship between architecture and site. It makes the building foremost an urban object, part of the boulevard and the public realm. In addition, Zumthor's building will be essentially facing itself across Wilshire, creating a kind of hall-of-mirrors context. Museum-goers would be able to look down on passing traffic.

These new conditions — a building spanning one of the world's most famous boulevards instead of a museum wing on an open, landscaped site edging up to a group of tar pits — would seem to require a new architectural approach from Zumthor, or at least a significantly modified one.

The building will open more generously to a new plaza on its western edge, facing the Broad Contemporary Art Museum and the Resnick Pavilion. But in terms of scale and materials — in terms of its basic architectural personality — the design awaits refinement. In general it has been left misshapen, like a piece of taffy, by the decision to stretch it across Wilshire.

It is hard to think of another location where an architect has staked a claim on both sides of a major Los Angeles boulevard. Frank Gehry and the developer Related Cos. have produced designs for a new retail and residential complex across Grand Avenue from Gehry's Walt Disney Concert Hall. Gehry has proposed a pedestrian bridge above Grand linking the two projects, but it would have a much more modest impact than the new LACMA design. A 2012 proposal for an addition to the Los Angeles Convention Center, never built, would have straddled Pico Boulevard.

There is still plenty of time for Zumthor to rethink the cladding and scale of the building and the details of its relationship to the roadway below. He is an architect who works deliberately. And this proposal is not meant to be final by any means.

At the same time, Zumthor's great skill, what made him attractive to Govan and the LACMA board in the first place, is a rare sensitivity to site and landscape. That talent was clear to see in the original plan, with its formal nods both to the tar pits and to abstract modernism.

It is tougher to spot in the updated version, which so far seems driven far more directly by political and urban-planning concerns than architectural ones.

Govan has his ducks in a row at City Hall, and credit to him for that. Now it's time to turn back to the design details of his bid to remake the Miracle Mile.

Times staff writers Abby Sewell and David Zahniser contributed to this report.